The "True Blood" and "Enlightened" actors portraying "hippie people who have been learning to be parents" also tell THR why Lily Kenna's performance as young Hazel moved them most.

There's more than one love story in The Fault in Our Stars.

Long before Hazel Grace Lancaster (Shailene Woodley) and Augustus Waters (Ansel Elgort) spark a new relationship after meeting in a cancer support group, Hazel's parents Frannie and Michael fell in love.

Played by Laura Dern (Enlightened) and Sam Trammell (True Blood), the Lancasters are "these sort of hippie people who have been learning to be parents by having, all of a sudden, a sick child," Trammell told The Hollywood Reporter at a recent Fault event.

Dern says she was thankful for the details present in John Green's best-seller, especially since the first day Dern and Trammell met was when they were shooting their Lancaster family portraits. "We had to make it 'family' instantly!" she laughed. In retrospect, she added, "So many people define collaboratively who that character's gonna be, and in any other experience I've had, I'm sitting with a costumer and saying, 'God, I don't know, would she wear that? I don't know.' But day one, we were saying, 'No, he wouldn't wear that! Where's the Patagonia? He wouldn't wear a business suit!'"

Trammell added, "We were lucky enough to both know how we met, and we had this shared biography that we had from the book. We all loved the book so much, so we were trying to protect it, and the fans watching it also want it to be protected."

Both actors noted that they drew from their own emotional vulnerabilities as parents with young children, but still cannot fathom the pain that comes with potentially losing a child to cancer. "It's one thing, as an actor, to consider heartbreak, addiction and longing and all the things we've gotten to portray on film, but, my God, consider something that you know so many people are walking through right now -- and it ain't a movie -- that just cracks your heart open," said Dern, who then turned to Trammell and said, "I remember crying with you when we had to do the scene where we're watching her go through her MRI; you're on the other side of the glass, and you can't be inside with them. The vulnerability of having to surrender your child to other professionals, waiting for diagnoses, and with doctors crudely going, 'You can't do this, you can't do that, you're too sick,' it's just so harsh -- and they're just doing their job, but it's so horrific."

"There was a vulnerability she had because she hadn't been in a movie before, and this is a big movie coming through," said Trammell. "You saw her sitting there with all the medical equipment, not really knowing what's going to happen. It really was a very similar thing that you'd think would happen with a child who didn't know what's happening with their disease."

Another sequence included having Kenna's head shaved -- by Dern. "She's a little girl who's so innocent and has never been in a movie, and she has beautiful long hair, and day one, I'm shaving her head, which was so brave of her," she said of Kenna, who donated her hair to Locks of Love, the nonprofit that provides hairpieces to children with long-term medical hair loss. "It was traumatizing to me, actually -- her mother was crying, and Lily was cool. Then, to get into those scenes and actually witness the oxygen and tubes and chemo drugs everywhere -- that palpable hospital experience -- I found those so traumatic." Trammell echoed, "A lot of her classmates had read the book, and she was going to have to go back to school, to seventh grade, with that haircut. It was a big sacrifice."

Still, both actors celebrated their on-set experience with Woodley, Elgort and Nat Wolff. "She carries such a purity of heart, and such a love of people and planet, and she's beyond genuine -- it's almost alien in a world of social media and lack of connection, to see someone her age require connectedness form everyone she meets.," said Dern of Woodley.

Trammell said of the trio, "They really have their heads on their shoulders, and they're wise beyond their years -- so much so that we really felt like their peers when just hanging out with them, with the exception of the references we would make to things like Caddyshack or Pearl Jam! They're all really smart. I have a lot of respect for them, and it's so nice to meet them on the way up -- I mean, they're already at the top. They're just really kind people."